Insulation for symmetrical cables

The conductors in the symmetrical cable are generally copper wires with a wire diameter of 0.25 mm to 1.4 mm. The diameter of the symmetrical cable used in the city is usually 0.4 mm to 0.9 mm; the diameter of the symmetrical cable used for long distance is usually 1.2 mm. Symmetrical cables with aluminum cores also appear in rare cases.
For the insulating material of symmetrical cables, it is divided into the following three categories:
1) Paper: Paper insulated cables are available in three forms: paper bag, pulp and rope. The rope insulation method is to wind the rope in a spiral shape on the wire, and then wrap it with 1 to 2 layers of paper tape.
2) Polyvinyl chloride: The dielectric constant and dielectric loss of polyvinyl chloride are relatively large, and it is limited to use on short-distance symmetrical cables. But the cables of this material are flame-retardant and can be used for indoor wiring.
3) Polyolefin and polystyrene: These two materials have good high-frequency performance and are often used in cables that transmit signals at the megahertz level or below. Polystyrene is often made in the form of ropes. And polyolefins are mostly in the form of solid, foam or foam skin.
In order to facilitate identification, different insulating layers are usually selected in different colors.